Issue 807 - Customizable Markdown options

[Marc-Andre-Rivet]

Fixes #807

Allows configuring markdown options at the table-level and will be compatible with future extensions if/when markdown support is increased.

Use the following value to open links in the current tab and get the behaviour as before #787

markdown_options={"linkTarget": "_self"}

[chriddyp]

maybe link_target to keep our property naming consistent?

[Marc-Andre-Rivet]

@chriddyp Will do. Requires mapping the API props to internal props for the Markdown library but no problem otherwise.